Centrifugal Fan / Blower - Changeover procedure for Incinerator application

[srinivasakumar]

Dear Members,

We are planning to install a spare blower ( centrifugal fan - API610) for our SRU Tail Gas incinerator. The control system obviously needs modification. We are trying to work out a scheme that will permit changeover from operating blower to standby blower with no interruption or flow rate change to the incinerator. If any of you have experience with these systems, can you advise a sequence? I would like to believe this is being routinely done with pumps in many process plants and someone will be able to suggest a scheme.

For information, the blowers have inlet flow measurement, inlet and outlet dampers, and discharge to a common header. please oblige.

 

[JJPellin]

You refer to a blower built to API 610 which is a pump standard. Are you referring to the combustion air blowers or the process blowers? What configuration are the blowers?

Johnny Pellin

 

[georgeverghese]

Since there will a low flow trip sensor on the air line to the incinerator, the first requirement would be to enable a large moment of inertia on the fan drive, so that its speed drops slowly on fan trip, thus keeping the air flow at above trip flow until the spare fan has picked up speed - that's a tall order. It also means a fast ramp up to full speed on the standby fan.

 

[ScottyUK]

If it is a controlled changeover then it should be possible to achieve this using variable speed control of the two motors such that the incoming motor ramps up in speed and as it starts to generate forward flow into the duct, the running motor ramps down to maintain overall constant flow.

If you are trying to maintain constant conditions after a trip event then that will be virtually impossible because the standby motor is reacting to the event and not running in anticipation of the event, unless you have the standby motor running so it is idling just below the speed where forward flow would occur - but that will almost certainly cause other problems.

 

[georgeverghese]

If you have a heavy speed reduction gearbox in the drive train, or a large flywheel, that will help with increasing rotational inertia, thus reducing speed decay on running fan trip. Else, see if you some auxiliary air supply that can be diverted here during the unplanned auto changeover.

 

[LittleInch]

API 610 blower - New one on me - please check.

As noted above there are two scenarios and I don't know which one you refer to.

1) Switch over from duty to standby as part of a planned operation / regular occurrence
2) Trip of the duty unit followed by standby unit coming to life.

Makes a big difference.

It all depends on the susceptibility of the system to small variances in flow. True "bumpless" transfer takes a lot of time and effort and requires some sort of flow control on the two units to swap duty over a certain time period ( say 30 seconds) and react to changes to maintain a fixed outlet flow. Not sure VFD would do the trick as the reduction in speed would affect the outlet pressure and hence overall flow rate. Might need control valves / dampers.

A lot of pump systems you just turn on the second pump, wait a few seconds as it gets to speed then turn off the first one. Bigger pipeline units you would probably simply start second pump against a closed valve, wait until it had got to full pressure and then simply open one valve as you close the other on the first pump. All depends on your downstream system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JJPellin]

I am not sure that a pump switching procedure would work. The configuration of the blowers may dictate what would work. One of our TGTU process blowers is a six stage, general purpose centrifugal blower running at 3600 rpm and directly driven by a general purpose, single stage non-condensing steam turbine. Another TGTU process blower is a single stage, overhung, special purpose compressor running at 7000 rpm and directly driven by a multi-stage, special purpose non-condensing steam turbine. The switching requirements for these two machines would be different.

If I was switching two general purpose pumps, I might consider two options. One, as suggested above, would be to simply start the back-up pump and shut down the main pump as soon as the back-up is developing pressure. If this was too much upset to the process, I would start the second pump blocked in and gradually work the valve open while gradually pinching down on the discharge of the main pump. This requires running both pumps dead-headed for at least a few seconds. I could not do that with our high speed, overhung process blower.

Depending on the configuration of your blowers and drivers, it might be possible to do some automation of the switching procedure for a planned switch. But, for a trip and auto-start scenario, it may be impractical. In any case, you need to provide more details about the service, the blowers and the drivers.


Johnny Pellin

 

[srinivasakumar]

Dear Members.

My apologies for the late response & Sincere thanks for your inputs..

The Fan is to API 673. It was a mistake in my original post.

it is direct drive. No VFDs. No gear boxes.

It is Switch over from duty to standby as part of a planned operation / regular occurrence.

There is inlet and outlet damper on both fans.

I recognize there will be bumps during switchover.

My question is:

1. I propose to start the standby blower, with minimum flow and progressively increase. While doing the reverse with operating blower.
2. But, at lesser loads, can the standby blower develop sufficient head to discharge into the common header? The operating blower is already at full head.
3. I am told the inlet damper flow control will mean outlet pressure will fall with reduced flow.
4. There is indeed a fan curve that shows the effect of varying damper openings. But it will help if someone could explain.

Please oblige.

 

[georgeverghese]

Does the fan curve show a surge line ? - if so, an exit line excess flow vent control valve will most likely be required to run the standby fan at some safe distance from the surge line while it waits for the running fan to stop. There is usually a limit to the extent you can throttle the suction damper.